In a conventional Frequency Division Duplexing (FDD)-based Multi-Input Multi-Output (MIMO) antenna system, there are relatively few antennae for a network device, and the measurement of a complete MIMO channel matrix is not an issue for a terminal. Usually, one antenna port is configured to each antenna so as to transmit a pilot signal for measuring CSI.
In a Long Term Evolution (LTE)-Advanced system, the pilot signal for measuring the CSI is called as a reference signal, including a Cell Reference Signal (CRS) and a CSI Reference Signal (CSI-RS). The terminal determines an optimal Rank indicator (RI), an optimal Precoding Matrix Indicator (PMI) and a Channel Quality Indicator (CQI) based on the measured CSI, and reports them to the network device through a feedback channel.
For a massive MIMO technique, a large antenna array, usually including hundreds or thousands of antennae, is deployed for the network device, so as to effectively improve a spatial resolution as well as a system capacity. When the antennae are arranged as a two-dimensional (2D) Uniform Rectangular Array (URA), it is able to achieve a very high three-dimensional (3D) spatial resolution (both in a horizontal direction and a veridical direction).
However, there is no scheme for the massive MIMO system to feed back and acquire the CSI.